Although tread-wear is theoretically possible through the transference of a thin surface layer from the tread to the pavement, the consensus of those who have investigated the process is that abrasion is the phenomenon involved. Abrasion partly results from "combing" of asperities in the road surface through the viscous rubber during tread-slippage. This action forms horizontal shear stresses and other stress distributions. When the stresses are sufficiently large, the rubber ruptures, particles are detached from the tread, and abrasion is said to have occurred.
There is evidence that abrasion also originates in fatigue failures of the surface rubber, particularly as affected by oxidative degradation of its component polymer. Here too, the shear and tensile strains at the tread/road interface are controlling factors. Other influences are the interfacial pressure, the tread temperature and the composition of the tread rubber. The interfacial pressures and shear stresses involved in abrasion appear differently on macroscopic and microscopic scales. As may be observed on glass surfaces, there are elastic instabilities which produce variations in shear in the rubber surface from point to point. There are also variations in interfacial pressure. This tendency is aggravated for sliding motions on all kinds of surfaces. Here viscous and inertial phenomena cause elastic waves to develop. High local stresses associated with these waves create elastic instabilities, which in turn lead to minute ruptures or regions of appreciable fatigue. The local ruptures serve as foci for both stress concentrations and slip discontinuities, and the process tends to be self-maintaining.
Stress variations and tearing in rubberlike materials furthermore depend upon dynamic conditions such as slip. Thus, it has been determined that the presence of slip promotes abrasion far beyond that expected from the magnitude of the macroscopic shear stresses alone. Accordingly, the presence of slip is a prerequisite for significant rates of abrasion.
The greater the interfacial pressure and interpenetration of the surfaces, the greater the horizontal stresses have to be for slip at an interface. This leads to the consideration of friction limits of the ratio of horizontal stress to pressure, above which slip takes place and below which it is absent. For the sake of simplicity, the existence of a single-valued effective coefficient of friction indicates the existence of slip. The rate of abrasion as well must bear some relationship to the interfacial pressure during slip. In the expectation that the complexities of the overall analysis will far outweigh minor transgressions in detailed form, the present invention is based upon a simplifying concept. That is, the abrasion rate is proportional to the macroscopic interfacial pressure present during slip.
The influences of tread temperature, road surface texture and tread rubber composition are viewed as conditions which control the viscous drag of road-surface asperties through the tread material and lead to rupture. Furthermore, the tread temperature and rubber composition also determine the rate of creep and deterioration of the tread rubber due to imposed stress concentrations. They also establish the rubber's threshold for microscopic failure. All of these influences are grouped into a single term which is a proportionality factor (a scaling coefficient) for the rate of abrasion. Arguments could be put forth for other kinds of mathematical relationships including interaction terms, etc., but what is used is a first order approximation, the accuracy of which has been tested through experimentation.
The statistical nature of abrasion during the road test is brought into the predictive procedure of treadwear. The conditions at which tires operate, and which affect tread-wear, vary from instant to instant during the road tests. Irregularities in the road surface cause suspension bounce and changes in deflection. Even on straight highways, there is the necessity for corrective steering maneuvers, and there is the normal tendency of vehicles to wander from one side of a lane to the other. In addition, wind drift, path curvature, braking, accelerating, tractive efforts on hills, etc., all affect wear of the tread. The accommodation of all these disturbances is the key to the method described hereinbelow.